All you need to do to create some fog is make a brush where you want
the volume of the fog to be and add a fog shader to it. from the sfx
category. If you want to change the color or thickness of your fog then
you should first read the shaders section.

Stuff you need to know
before making your own fog shader

If
you have read the shaders section then to create your own custom fog do
the following things in the following order. remember in the file fogparms
( # # # ) ### the first 3 #'s are R, G, and B and the last ###'s
default is usually 800 this is the thickness of the fog. If you want fog
so thick you cant see 1 unit in front of your face set this number to 0,
if you want it as thin as air so you can barely see it set it to 8000. If
I wanted to create a really thin blue mist type of fog I would make the
line say: fogparms ( .7 .7 1 ) 5000 that is all you need to
know really.

Making your own fog
Shader

If
you have read the shaders section then to create your own custom fog do
the following things in the following order.

Make sure you have
EditPad installed, if you don't have it download it here.

Make a new .shader file
in your baseq3\scripts directory, I named mine bluefog.shader. Just
open up EditPad and do a new file and save it baseq3\scripts\bluefog.shader.

Now use EditPad to open
up baseq3\scripts\shaderlist.txt, and at the very bottom type in
bluefog, or whatever your shader's name is.

Make a new directory
baseq3\textures\bluefog\

Open up baseq3\scripts\sfx.shader,
use edit/find and replace then for text to find type in
fog_intel and click find. Now you should see: (fog_intel
will be highlighted)

Open up your .shader file you saved and copy and paste
the above into it. now you can change your colors and strength. in the
fogparms line. Mine was: fogparms
( .7 .7 1 ) 5000

Change the top of the
file, the part that says: textures/sfx/fog_intel to: textures/bluefog/bluefog

Save your file to baseq3\shaders\bluefog.shader.

Now change the: textures/bluefog/bluefog part
back to the way it was: textures/sfx/fog_intel And now save the
file to baseq3\textures\bluefog\bluefog.shader.

When you open up the Radiant program you should have a
new line under textures that says bluefog. When you click on it you
should see a fog icon that says: [bluefog] on the top.

Create a new brush where you want your new bluefog to
be and give it the new bluefog shader.

Compile the map and run it and you should see your new
bluefog mist. If you want to change the fog colors and density you can
do so, just leave the arena and then open up both of your .shader
files and change the numbers and save them both then reload your map.
That's right you do not need to compile your map again! Just reload
it!